Method of providing an optical data carrier with identity information

ABSTRACT

When manufactured, an optical data carrier is provided with digital information, which is written and stored in accordance with at least one previously defined encoding method for error correction and which may be read and decoded at a later stage by means of an optical reader with correction of errors in accordance with said encoding method. A predetermined number of logical symbols or bits are selected at predetermined positions in the digital information to be stored on the data carrier. A set of logical errors are intentionally created by replacing the selected symbols with corresponding symbols in a predetermined code sequence representing the identity of the data carrier. The logical errors are of such a type as to be normally corrected by the optical reader, when the data carrier is read and/or copied. The presence of at least a subset of the set of logical errors during reading without error correction of at least part of the digital information on the disc is interpreted as evidence of the data carrier having an identity determined by the set of logical errors.

TECHNICAL FIELD

The present invention relates to a method of providing an optical datacarrier with identity information, said data carrier when manufacturedbeing provided with digital information, which is written and storedaccording to at least one previously defined encoding method for errorcorrection and which may be read and decoded at a later stage by meansof an optical reader with correction of errors in accordance with saidencoding method.

DESCRIPTION OF THE PRIOR ART

Compact discs (CD) are a common type of optical data carriers accordingto the above. Compact discs are provided in various forms, such asCD-Audio (containing for instance music), CD-ROM (secondary computerread-only memory), CD-I (interactive CD), and Photo-CD (containingdigital photographs). In the context of this document the term compactdisc will be used in a broader sense relating to any kind of opticalcompact discs—also such types not explicitly specified in this document.In particular, it should be understood that the discussion in thisdocument is applicable also to the high density compact disc standardcalled DVD (Digital Video Disc), which was recently introduced byToshiba, Japan.

Compact disc applications are today taken for granted in our dailylives. We listen to music on compact discs at home as well as in ourcars. When working with computers we often use information stored onCD-ROM. An increasing number of commercial software products arenowadays available on CD-ROM, such as utility programs (e.g. wordprocessors and spread-sheat programs) or entertainment programs (e.g.computer games). Some of us choose to store our vacation pictures onPhoto-CD rather than keeping our pictures as paper-printed photographsor slide photographs.

As a consequence of the use of compact discs having become increasinglypopular, it has become desirable to provide compact discs with identitymarking. It may for instance be convenient to provide a compact discwith information regarding, e.g., manufacturer, author, article number,etc. Another example is preventing the production of illegal copies,considering that is has become increasingly attractive to lessscrupulous individuals to copy and plagiarize commercially availablecompact discs products, CD-Audio and CD-ROM in particular. In certainparts of the world a complete industry has been developed tomass-produce illegal copies of copyright-protected compact disc productsand to sell these illegal copies to prospective buyers. Since thelegitimate author or owner of the property will get no compensation forhis work and his costs, there is a risk of a cultural as well as atechnical impoverishment, should the illegal copying and plagiarizing bepermitted to continue its explosive growth.

EP-A-0 637 023 discloses a method of protecting an optical data carrier,such as an optical disc, from illegal copying by providing it with aplurality of irregular storage pits, which are automatically correctedduring normal copying and which, as a consequence, will not be copiedonto the disc copy. The irregular pits have a physical shape deviatingfrom a perfect or normal shape. Furthermore, the irregular pits arearranged in an irregular sequence, which generates a tracking errorsignal indicative of the presence of the irregular pits. In addition,the irregular pits may be detected by the deviations in symmetry for theanalog HF-signal waveform. By scanning the disc for such irregular pits,it is possible to determine whether the disc is a legal and originaldisc (containing said irregular pits), or an illegal copy (lacking saidirregular pits).

U.S. Pat. No. 5,513,260 relates to a similar method and apparatus forprotecting various recording media. An authenticating signature isrecorded on the media by radially position modulating the writing heador varying the specific data blocks.

A drawback of a copy-protection principle according to EP-A-0 637 023 isthat additional hardware or equipment is required for creating thephysical irregularities in the storage layer of the disc. Furthermore,since these irregularities are of a physical nature (i.e., the physicalshape or arrangement of the irregular pits is different from that of anormal pit), an infringer or plagiarist may be capable of locating thosepositions on the disc, in which the copy-protecting information (theirregular pits) are located, whereupon the irregularities thus locatedmay be transferred to, or recreated on, the disc copy.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved identitymarking for an optical data carrier in such a way, that only authorizedusers are able to read identity information stored on the data carrier,while said identity information will be hidden from and unavailable toall other users. One embodiment of the invention aims at rendering itpossible to determine the authenticity of an optical data carrier, suchas a compact disc, in order to be able to decide, whether a certain dischas been produced by the rightful manufacturer, or by an unauthorizedperson or manufacturer.

Such an improved identity marking principle, which does not suffer fromthe disadvantages described above, may be obtained by providing the datacarrier with intentionally arranged errors on a logical level (bit orsymbol errors, e.g. errors among the digital ones and zeroes stored onthe disc), rather than a physical level.

Thus, the object is achieved by a method with features according to thecharacterizing part of the appended independent patent claims. Preferredalternatives to the method according to the invention are specified inthe dependent claims.

DETAILED DESCRIPTION OF THE INVENTION

The digital information on a compact disc is stored as pits and planeareas (so-called lands) in a layer of plastic or the like. The pits andthe plane areas represent the binary symbols “one” and “zero”,respectively, in the digital information. The contents of the compactdisc are read by means of a laser beam passing through the informationlayer of plastic or the like and reflecting in a reflective layer ofmetal or the like. The reflected laser light is converted by a photodetector into an electrical signal, which is digitalized.

Since the compact disc is storing large amounts of information on asmall surface (a conventional compact disc is capable of storingapproximately 650 MB of information, which is equal to more than fivebillions of binary ones and zeroes), extensive measures have been takento minimize the risk of read errors due to irregularities in the disc,scratches, grease stains, signal noise, etc. When the standard for thecompact discs of today was established during the seventies (see forinstance Philips “Red Book”, “Yellow Book”, Green Book”, and “OrangeBook”), it was decided to use error-correcting block encoding for thedigital information on the compact disc.

Reed-Solomon-type block codes are used for compact discs. If an uncodeddata sequence contains K symbols and an encoded data sequence contains Nsymbols after encoding, then K symbols in the code sequence may beregarded as information symbols, while the remaining N−K symbols in thesequence are parity symbols. These parity symbols add redundancy to theencoded information, and thanks to this redundancy symbol errors may bedetected and corrected, respectively. For a block code to be able tocorrect all error patterns e with a certain so-called Hamming-weightw_(H)(e), where w_(H)(e) is equal to the number of symbols≠0, it is ageneral fact that the number of parity symbols must fulfil the conditionN−K>w_(H)(e)−1.

Reed-Solomon block codes are described in for instance “Fundamental ofConvolutional Codes”, R. Johannesson and K. Zigangirov, The Institutionof Information Theory, Lund Institute of Technology, 1992, or in“Bit-serial Reed-Solomon Encoders”, E. R. Berlekamp, IEEE Trans. Inform.Theory, IT-28:869-874 (1982), and hence these codes are not described inmore detail now.

The compact disc information is encoded in two levels (an outer code C1and an inner code C2, respectively). When reading the information, theinformation is correspondingly decoded in two steps by letting read datapass through a first and a second decoder. Each of the decoders is ableto correct double-symbol errors, and thanks to the two-dimensional codearrangement a compact disc reader is able to correct also rather seriousread errors.

To copy a compact disc the following usually takes place. Theinformation on the original disc is read by a compact disc reader anddecoded in two levels according to the above. The decoded digitalinformation is then fed to a writing device for recordable compact discs(CD-WORM; “Write Once Read Many”), where the information is encoded intwo levels before it is stored on the copy disc.

According to the invention an optical data carrier, such as a compactdisc, is provided with identity information by intentionally providingthe disc during the manufacturing process with logical errors, i.e. bitor symbol errors in the digital information stored on the disc,according to a predetermined pattern. The errors are only as severe aswhat may be corrected by the decoders of a compact disc reader, meaningthat a consumer will virtually never notice or be disturbed by the factthat the disc has been intentionally provided with said errors. If thedisc is illegally copied according to what has been described above, thecompact disc reader used in the copying process will correct theintentionally arranged errors, before the copied information is storedon the copy disc. Hence, an illegal copy will not be provided with theidentity-indicating errors of the original disc.

A preferred form of the method according to the invention will now bedescribed. Before the manufacturing of the compact disc—or rather beforethe manufacturing of the template (so-called stamper) used for theprinting of the compact discs—some bits or symbols are selected amongthe encoded digital information to be stored on the disc. These bits,which are preferably not located adjacently on the disc but are ratherdistributed throughout the entire disc or parts of the disc, are thenreplaced bit by bit by the corresponding bit in a code word representingthe identity of the disc. For instance, assume that the original bits(which have been selected in a certain way according to what will bedescribed below) may be represented by:

11010011

and that the code word may be represented by:

01110001.

Thus, in the example above, where only 8 bits are used for simplicityreasons—a lot more bits are used in practice the code word “01110001” iswritten bit by bit at preselected positions in the digital informationon the disc. Due to the fact that the code word is different from theoriginal information, an error pattern is achieved, which is equal tothe bit-wise difference between the original information and the codeword.

The information, which according to the above has been modified tocontain the intentional error pattern, is stored on every legallymanufactured copy of the compact disc in question. Since the errors arenot placed adjacently but rather in a distributed way according to thepredetermined pattern, the errors will be corrected when the contents ofthe disc are read during normal play-back. The bit errors will becorrected automatically also when the information on the original discis read in order to be transferred to illegal copies. Hence, thesecopies will not comprise the intentional bit errors of the originaldisc.

The number of bits as well as their positions on the disc may be chosenin many different ways. Preferably, however, the bits are chosenaccording to a predetermined keycode available only to authorizedpersons or companies, e.g. the manufacturer or the copyright proprietor.This keycode may for instance indicate to use every n:th bit position,where n=1, 2, . . . m, thereby giving a code word of n·m bits. As analternative more advanced keycodes may be used, stating for instancethat bits number 13, 893, 12415, 23880, etc. are to be used. The numberof possible keycodes are more or less infinite. It shall be noted thatit is impossible for anybody without knowledge of the keycode to decidewhich bits among the several billion bits on a compact disc thatconstitute the identifying error pattern.

For increased security and reliability the error pattern may be repeatedat several locations on the disc. Furthermore, the distributed code wordmay in itself be encoded according to an error correcting code, therebyensuring a correct identification even if errors should occur during thereading of individual bits. In reality, is is impossible to avoidoccasional errors during the reading of the digital information on adata carrier—and indeed, this is the reason for using error-correctingencoding methods. Obviously, errors may occur also when reading theparticular bits representing the identity-indicating code word/errorpattern. However, provided that the code word is chosen long enough,i.e. containing a large number of bits, such read errors have no impacton a secure and reliable identification. For instance, if two of thebits in a code word of 10,000 bits would be read erroneously(corresponding to an error rate of 210⁻⁴), 9,998 correctly read bitsanyhow remain that indicate the correct identity information of the datacarrier, provided that they match the corresponding bits in thepredetermined code word.

The identity information provided on every legally manufactured datacarrier may indicate the manufacturer, site or machine being used forthe production of the data carrier (or alternatively record company,software company, etc). Consequently, every compact disc manufacturermay be given an identity represented by an error pattern being uniquefor the manufacturer. The stamper used when manufacturing the compactdiscs is provided with the unique error pattern, and consequently alldiscs manufactured by means of the stamper will contain the errorpattern. Hence, manufacturer A may for instance be assigned the errorpattern A′, whereas manufacturer B may be assigned error pattern B′, andso on. It will then be possible to determine not only the authenticityof any given compact disc but also its origin.

To be able to check the authenticity of the disc and to determine itsidentity according to the above it is obviously necessary to read thedisc on a low level basis by means of equipment capable of readingindividual bits at given positions on the disc without carrying out anynormal error correction. It is believed to be within the qualificationsof a man skilled in the art to design such an apparatus by means ofpreviously known standard components, such as precision lasers andhigh-sensitive photodetectors.

Within the scope of the invention, as defined in the appended patentclaims, the method according to the invention may be performed inalternative ways not described above, which is readily realized by a manskilled in the art. In particular it is noticed that the logical errorpatterns may be done in various different ways, as long as they arecorrected during normal play-back or reading of the compact disc, and oncondition that they may still be detected when the disc is examined on alow level basis according to the above.

What is claimed is:
 1. A method of providing an optical data carrierwith identity information, said data carrier when manufactured beingprovided with digital information, which is written and stored accordingto at least one previously defined encoding method for error correctionand which may be read and decoded at a later stage by means of anoptical reader with correction of errors in accordance with saidencoding method, and said data carrier being intentionally provided witha set of identity-representing errors or irregularities, which arenormally corrected by said optical reader during reading and/or copyingof the data carrier, characterized by the steps of: a) prior to themanufacturing of the data carrier selecting a predetermined number oflogical symbols or bits at predetermined positions in the digitalinformation to be stored on the data carrier, and b) creating said setof identity-representing errors on a logical level by replacing theselected symbols with corresponding symbols in a predetermined codesequence representing the identity of the data carrier, and storing thedigital information thus modified on the data carrier, wherein thepresence of at least a subset of said set of logical errors duringreading without error correction of at least a part of the digitalinformation on the disc is interpreted as an evidence of said datacarrier having an identity determined by said set of logical errors. 2.A method according to claim 1, characterized in that the lack of saidset of errors during reading without error correction is interpreted asan evidence of said data carrier being an illegally manufactured copy.3. A method according to claim 1 or 2, characterized in that said errorcorrecting encoding method is constituted by Reed-Solomon blockencoding.
 4. A method according to any preceding claim, characterized inthat the optical data carrier is a compact disc (CD).
 5. A methodaccording to any preceding claim, characterized in that the optical datacarrier is a compact disc of the type called DVD (Digital Video Disc).6. A method according to any preceding claim, characterized by theadditional step of: c) checking the authenticity of the data carrier byreading the information contents of the data carrier at saidpredetermined symbol positions without error correction, puttingtogether the symbols read from the data carrier, comparing these symbolswith said predetermined code sequence, and stating that the data carrieris authentic,. if a correspondence exists between all symbols or amajority thereof.